In order to improve thrust and fuel consumption of gas turbine engines i.e. the thermal efficiency, it is necessary to use high compressor pressures and higher combustion temperatures than have conventionally been used. Higher compressor pressures give rise to higher compressor outlet temperatures and higher pressures in the combustion chamber giving rise to the combustor chamber experiencing much higher temperatures.
There is, therefore, a need to provide effective cooling of the combustion chamber walls. Various cooling methods have been proposed including the provision of a double walled combustion chamber whereby cooling air is directed into the gap between the chamber walls thus cooling the inner wall. This air is then exhausted into the combustion chamber through apertures in the inner wall. The inner wall may also comprise a number of heat resistant tiles. Constructing the inner wall from tiles has the advantage of providing a simple low cost construction. Combustion chamber walls which comprise two or more layers whilst being advantageous in that they only require a relatively small flow of air to achieve adequate cooling are prone to some problems. These include the formation of hot spots in certain areas of the combustion chamber wall and the combustion chamber. Prior art proposals to alleviate this problem include the provision of raised lands or pedestals on the cold side of the wall tiles. Reference is hereby directed to GB Patent no. 2 087 065. These lands or pedestals serve to increase the surface area of the wall element thus increasing the cooling effect of the air flow between the combustor walls. Compressor delivery air is convected through pedestals on the `cold face` of the tile and emerges as a film directed along the `hot` surface of the following downstream tile.
The provision of such lands is also accompanied by inherent problems. For example localised overheating may occur behind obstructions such as mixing ports or adjacent to regions where near stoichiometric combustion gives rise to high gas temperatures (hot streaks). There is no provision for enhanced heat removal, either locally to remove hot spots or to alleviate more general overheating towards the downstream end of the tile. Overheating may occur downstream of the mixing ports since the protective wall cooling film is stripped away by the transverse mixing jets. Where design requirements have dictated a relatively long tile the cooling film quality towards the downstream edge of the tile may be poor and lead to overheating.